Video DownloadHelper is the best at what it does that I've used (been using it for several years now). It's fast, and extremely easy to use, and so far I've been able to download every video that I wanted to download with it. I like being able to accurately monitor download rate and progress, and being able to download as many videos simultaneously as I've needed to. I've not had any problems with it so far. I consider it to be one of my most essential extensions. Kudos and thanks to the developer for providing this very useful Firefox extension.